Bug 1057604 - Use the hidden attribute for the save button r=dkuo

While title-player and title-done did proper use of their "hidden"
attribute to be hidden via CSS rule setting "display: none", the save
button was not part of this list. Thus, setting the "hidden" attribute
to true had no impact on the visibility of the element.
Merge pull request #23581 from lissyx/bug1057604

Bug 1057604 - Use the hidden attribute for the save button r=dkuo